In recent years, accompanying the speeding up of information communication, the wiring board is used in a high frequency region of GHz band or more and demanded to be reduced in the transmission loss. To satisfy this requirement, the wiring board is produced by forming a conductor layer composed of a metal having a low conductor resistance and a low melting point, such as silver and copper, on a ceramic substrate having a relatively low dielectric constant. With the progress of high-density packaging of a circuit, copper is used for the conductor layer so as to prevent migration. The ceramic substrate having printed thereon copper as the conductor layer is fired in a wet nitrogen atmosphere (in a mixed atmosphere of water vapor and nitrogen gas) so as to obtain a wiring board reduced in the transmission loss while preventing the oxidation of copper.
Generally, for producing a wiring board, a slurry is prepared using a ceramic raw material powder and an organic binder, a solvent or the like and formed into a ceramic green sheet by a sheet-forming method such as doctor blade method. Thereafter, a wiring pattern is printed on the ceramic green sheet by using a copper paste and dried. Subsequently, the ceramic green sheet is debindered at a temperature of hundreds of ° C. in a mixed atmosphere of water vapor and nitrogen gas to remove organic components contained in the copper paste and ceramic green sheet and then fired by elevating the temperature to nearly 1,000° C. or more, thereby producing a wiring board.
With the progress of high-density packaging and multifunctionalization of equipment, a semiconductor element such as transistor and diode, heat radiation parts, terminals and various circuit components are mounted and therefore, the adhesive strength of the conductor layer formed on a wiring board is demanded to have higher reliability.
Particularly, in a wiring board having mounted thereon a semiconductor element such as transistor and diode, the semiconductor element generates heat due to input signal to cause elevation of temperature and this causes deterioration in the properties of semiconductor element or other circuit component mounted on the circuit board. Therefore, heat radiation is important. For this purpose, in the wiring board, a radiator is connected through a conductor layer in many cases and the conductor layer formed on the wiring board is required to have an adhesion strength high enough to cause no bulging or separation against heat load.
As a technique for preventing the bulging or separation of the semiconductor layer formed on a wiring board, a method of using a thick film copper conductor ink as the copper paste described in JP-A-1-128488 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) is known.
In the thick film copper conductor ink described in JP-A-1-128488, by taking notice of the fact that an organic vehicle contained in a conductor ink generates a gas in the firing step to cause bulging or separation of the conductor layer, a devitrifying glass flit selected from a zinc-calcium aluminum silicate glass flit, a zinc-magnesium-barium-aluminum silicate glass flit and a mixture thereof, and a specific oxide such as bismuth oxide and cadmium oxide are added to the copper paste to thereby suppress the generation of gas and prevent the separation or bulging of conductor layer.